In a gear box for an automobile or other machine, a breather plug in connected to a housing of the gear box. The plug is provided with a deflector of tubular shape which is connected to the housing and extend to a space in the housing. When the pressure in the housing is increased due to a heat expansion of lubricant oil stored in the housing, a valve mechanism incorporated in the plug is opened for venting the space in the housing to the atmosphere, in order to prevent the pressure in the housing from being highly increased. However, in breather plugs of the prior art, wherein the deflector is contacted with the flow of lubricant formed along the inner surface of the gear box, a ventilation operation cannot be fully effected due to an air space formed downstream of the deflector. As a result of this, a pressure in the housing is easily increased, and there is a large possibility of lubricant leakage.